ATM Telecommunications Systems and Method for Routing Narrow Band Traffic

ABSTRACT

A telecommunications system comprises an asynchronous transfer mode (ATM) network having uncommitted bandwidth, and a plurality of adaptive grooming routers (AGR) coupled to the network. The AGRs comprise a group adapted to function as a virtual transit exchange whose fabric and control are distributed over the group. The visual comprising the AGRs incorporates independent connection control and call routing functions and has means for determining the current system status whereby to set up narrow band connections across the ATM network based on that status determination.

RELATED APPLICATION

This application is a division of co-pending U.S. patent applicationSer. No. 11/055,787, filed Feb. 10, 2005, which is a reissue of U.S.Pat. No. 6,519,257.

BACKGROUND OF THE INVENTION

This invention relates to digital communications systems and inparticular to systems embodying asynchronous transfer mode (ATM)technology.

The asynchronous transfer mode (ATM) technology is a flexible form oftransmission which allows any type of service traffic, voice, video ordata, to be multiplexed together on to a common means of transmission.In order for this to be realized, the service traffic must first beadapted typically into 53 byte cells comprising 5 byte headers and 48byte payloads such that the original traffic can be reconstituted at thefar end of an ATM network. This form of adaptation is performed in theATM adaptation layer (AAL).

As telecommunications networks increase in complexity and carryincreasing volumes of traffic, the current procedures for setting upconnections between subscribers are limiting the performance of thesenetworks. In particular, congestion may be caused by attempting toconnect to a subscriber who is already busy, or by attempting to choosea route through an already congested part of the network. Thus equipmentand resources can be wasted in attempts to set up calls which cannot becompleted. A further problem is that of scalability. As the networkexpands to accommodate increased traffic and a larger number ofsubscribers, there is an increasing need to facilitate integration ofnew equipment into an existing network without simply increasing thecongestion problem.

SUMMARY OF THE INVENTION

The object of the invention is to minimize or to overcome thesedisadvantages.

According to one aspect of the present invention there is provided adistributed telecommunications exchange system having independent callrouting and connection control for setting up connections across thesystem.

According to another aspect of the invention there is provided atelecommunications system, including an asynchronous transfer mode (ATM)network, and a plurality of adaptive grooming routers (AGR) coupled tothe network, wherein the AGRs comprise a group adapted to function as asingle distributed or virtual transit exchange whereby in use to set upnarrow band connections across the ATM network.

According to one aspect of the present invention there is provided adistributed telecommunications exchange system having means fordetermining the current status of the system whereby to effect routingof narrow band traffic across the system.

According to another aspect of the invention there is provided a methodof communicating resource availability to maintain performance of adistributed exchange system under overload conditions.

According to a further aspect of the invention there is provided amethod of routing telecommunications traffic in a system including anasynchronous transfer mode (ATM) network having uncommitted bandwidth,and a plurality of adaptive grooming routers (AGR) coupled to the ATMnetwork, which AGRs comprise a group adapted to function as a virtualtransit exchange whose fabric and control are distributed over thegroup, the method including determining the current system statuswhereby to set up narrow band connections across the ATM network basedon that status determination.

The technique provides for the separation of call routing and connectioncontrol together with the advertising of the system status. This ensuresa wide range of scaleability so that the application of dynamic trunkingtechnology provides scaleability in a traffic sense. Further, theseparation of call routing and connection control provides a distributedcomputing environment resource. Because the distributed exchange managesits own internal traffic, effectively it provides means for balancingthat traffic to the fabric and makes its own internal routing decisions.

Reference is here directed to our co-pending applications Nos.9410294.4, 9410295.1, 9411944.0 and 9502552.4 which relate toarrangements and methods for handling narrow band traffic in an ATMcommunications network.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates an AGR network system that is the preferredembodiment of a large distributed exchange system;

FIG. 2 illustrates signaling separation in the network of FIG. 1:

FIG. 3 illustrates the provision of advanced services in the network ofFIG. 1;

FIG. 4 illustrates the principle of dual homing and diverse path routingexternal narrowband exchanges to the distributed exchange system;

FIG. 5 illustrates the connection of the large distributed exchange to ahierarchy of narrowband exchanges;

FIG. 6 illustrates an overview of handling a connection set-up requestand the roles of call routing, connection control and voice-pathconnection termed wormholes;

FIG. 7 illustrates the principle of using existing N-ISUP signaling asthe means for routing calls across the distributed exchange system atthe Call Routing layer;

FIG. 8 illustrates the principle of controlling routing and congestionin the Call Routing and Connection Control layers;

FIG. 9 shows a AGR protocol stack for the network of FIG. 1;

FIG. 10 illustrates a means for specifying a speech path across thedistributed exchange system;

FIG. 11 illustrates the functional topology of the physicallydistributed exchange system;

FIG. 12 illustrates an arrangement for determining available trunkcapacity on a Virtual Trunk Route (VTR);

FIG. 13 illustrates an arrangement for determining a Virtual TrunkGroup's (VTG) capacity;

FIG. 14 illustrates an arrangement for advertising congestion on aremote node's VTG in the distributed exchange system;

FIG. 15 illustrates an arrangement for advertising congestion on aremote node's VTR in the distributed exchange system;

FIG. 16 illustrates an extension of the arrangement of FIG. 13 forspecifying available voice band processor capacity;

FIG. 17 illustrates a preferred signaling process to create a speechpath between AVJs in the network of FIG. 1;

FIG. 18 illustrates the information content required to establish orremove a narrowband connection;

FIG. 19 illustrates a method for tracing a narrowband connection throughthe large distributed exchange from a trunk circuit; and

FIG. 20 illustrates the overall protocol for establishing a connectionend to end in the layers Call Routing and Connection Control and atphysical layers;

DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to FIG. 1, this depicts the functional architecture ofthe Adaptive Grooming Router (AGR) network system. The AGR works on theprinciple of a single fabric switch (SFS) incorporating a novel devicean adaptive virtual junctor (AVJ), that adapts narrowband traffic to andfrom the ATM adaptation layer e.g. (AAL-1), and also provides the meansto time switch narrowband channels. Using a standard ATM switchingfabric, the AVJ allows a fully scalable narrowband exchange to be built,by communicating through the switch fabric using a fully interconnectedmesh of dynamically sized trunk groups or virtual channel junctors(VCJ), which gives a non-blocking space switching performance. The AGRextends the scale of this principle to the wide area by usingdynamically sized Virtual Trunk Groups VTGs to construct a fullyinterconnected mesh of AGRs that can dynamically adapt to thecommunity-of-interest in the wide area network. The network of AGRs istermed an AGR network system, and functions as a large distributednarrowband exchange, that at a minimum can replace a single transitexchange, and at a maximum replace the existing trunk/transit layers ofan hierarchical narrowband exchange network. The term Adaptive GroomingRouter (AGR) is used since the node grooms traffic to/from localexchanges according to instantaneous demand for a trunk/transitconnection. The VCJs and VTGs function on the principle of dynamicstructured data transfer (DSDT) to provide a end-to-end narrowbandconnection capability, and the ability to perform path tracing.

The capacity of a single AGR node is dependent on the size of the hostATM switch, the number of connected AVJ devices and the capacity of theAVJ device itself. Further, the AVJ device can be dedicated to performeither a trunking function (termination of synchronous narrowbandcircuits) or a grooming function (generation of traffic groups fortransmission across the WAN) equivalent to a tandem switching functionof NB networks. Therefore both the overall capacity of the node and themix of traffic (intra and inter node) can be dimensioned according toneed. For example, consider a node built using 8,000 channel AVJdevices, hosted to a 10 Gb/s ATM switch. A total of 11 fullyinterconnected AVJ devices with four dedicated to trunking and 7 togrooming yields node with a total of 70,000 circuits (32,000 TDM trunksand 28,000 WAN trunks). In this configuration the node can terminate upto 28,000 Erlangs of traffic from the synchronous local exchange networkwith over 80% of this traffic capable of being carried across the WAN.If however, the amount of WAN traffic is less, then one or more of thegrooming AVJ devices could be sacrificed for a further trunking AVJ thusincreasing the amount of synchronous trunking capacity.

For AGR network resilience, a minimum of two VTGs will interconnect allof the nodes within a network. The capacity of the routes can bedimensioned according to the communication demand—high capacity VTGsconnecting high traffic routes, 47 trunks and up and low capacity VTGsconnecting low demand routes 6-46 trunks. In this manner the overallgrooming capacity of a node can be flexibly dimensioned to connect toeither a very large number of nodes (with modest traffic on each route)or to a lower number of nodes (but with a large amount of inter-nodaltraffic or a mix of both. As an example, consider the node describedabove, with 28,000 inter-nodal trunks. In one configuration this couldsupport 178 similar nodes each node interconnected by two VTGs with anaverage capacity of 80 trunks (the capacities of the individual VTGs canof course dynamically increase and decrease according to demand). Inthis manner a network capacity of over 5,000,000 narrowband trunks canbe supported with over 80% of this traffic able to traverse the WAN atany one time.

FIG. 1 also shows the manner in which the AGR node provides e.g. N-ISDNSS7 signaling capability and call routing.

The arrangement provides a means for establishing, maintaining andremoving connections in an AGR network. The AGR network system iscontrolled from a proxy call routing server in an ATM network to providenew services to users in the network, stemming directly from theaforementioned separation of call routing from connection control whichis illustrated in FIG. 2 which illustrates the manner in which adaptingsignaling physical layers to ATM allows evolution from STP basedsignaling to signaling as an ATM network service.

FIG. 3 illustrates the principle of an AGRNS that provides a connectionengine, controlling narrowband connections between endpoints specifiedby the local Call Routing subsystem, or by a proxy server. Therefore theproxy server may implement signaling systems that go beyond thecapabilities of the N-ISDN SS7 signaling subsystem in the AGR node, andprovide for example multimedia services and control thepoint-to-multipoint hardware capability of the AGR. In the exampleillustrated, the D channels of ISDN Primary Rate Access are routed to aframe processor located in the ATM network, accessed by a VirtualChannel Connection. The frame processor takes HDLC frames and adaptsthem to/from AAL-5. The D channels may carry HDLC frames that areadapted to the broadband MTP-2 protocol Signaling ATM Adaptation Layer(SAAL) and then routed to the proxy server. The HDLC frames may containany layer 3 or higher layer of the OSI model as signaling messages fordirect interpretation by the software within the proxy server. Thesemessages are translated into commands that use the ApplicationsInterface of the connection control of the AGR node or are formed intoSS7 messages or any other suitable proxy signaling protocol that can beinterpreted by the Connection Control layer of the AGR software toinstruct it to control narrowband connections between specifiedendpoints. By separating the service aspect from the connection aspectin this manner, new services may be made available in the network.

Referring now to FIG. 4, this illustrates the principle of dual homingof narrowband exchanges to the AGR network system. In hierarchicalnarrowband transit networks, a local exchange in the lowest tier mustprioritize its traffic across many small routes depending ongeographical destination of a call and traffic congestion levels. Sincethe AGR network system performs a grooming and dynamic handling oftraffic demand, the local exchange can now simply “hook on” to the AGRnetwork system using a large voice route. The large voice route hasincreased Erlang efficiency, and providing two large voice routesprovides a degree of resilience if the traffic is balanced across thetwo routes. The AGR network system can also make use of the dual homingfacility to balance traffic internally, since there is a choice ofdestination AGR when communicating with any dual homed narrowband localexchange.

FIG. 5 illustrates the connection of the AGRNS into an existingnarrowband network hierarchy of exchanges. In such a narrowband networkit is traditional to provision the network to have a choice of routeswith priority given to the most direct route between exchanges. Howeverthe routes will be sized such that they experience a differing degree ofblocking probability that decreases with decreasing priority, such thata backbone route via exchanges in the highest tier has a much lowerblocking probability than routes directly between local exchanges. TheAGRNS may be deployed to replace just one such exchange in tiers 2 or 3,or to provide a backbone narrowband network using ATM transport thatcould replace part or all of the narrowband transport and exchanges.This would ultimately form a single tier transit network. Consequently,the AGRNS must in general be able to cope with the prioritization ofvoice routes that connect it to the narrowband transit hierarchy. Thesevoice routes would suffer congestion because of occupancy and alsoexchange processor activity, as described in the ITU White BookStandards on SS7. Internally the AGRNS must cope with the sharedutilization of the ATM transport and with the blocking caused byfragmenting its grooming capacity across a number of physical devices.The load balancing arrangement shown in FIG. 5 exploits the dual ormulti-homed connectivity in the narrowband network to even out trafficoccupancy on external voice routes and internal trunk groups so as delaythe onset of congestion and maintain carried traffic levels underoverload conditions by rejecting offered traffic at the periphery of theAGRNS.

Reference is now made FIGS. 6 to 20 which illustrate stages of theconnection set up in the network of FIG. 1 in response to a set uprequest. The diagram of FIG. 6 shows two separate functions, callrouting and connection control. FIG. 6 also shows the stages ofprogression of the call. The incoming initial address message (IAM),which may comprise the initial address message of the number 7 signalingsystem, goes to call routing which makes decisions based on a digitanalysis by using as many digits as it needs to determine the outgoingvoice route or set of voice routes in the narrowband network (see FIG. 2b) and it may, if necessary, wait for other signaling messages. Oncecall routing has chosen the candidate set of voice routes it can thenmake decisions based on static prioritization of those voice routes.I.e. a voice route through a local exchange can be used in preference toone through a transit exchange, which preferably is used less often. Thecall routing then makes an assessment of congestion on those voiceroutes. This could either be a total circuit occupancy of the voiceroute or it could be congestion associated with an exchange and theprocessing power it has available to handle signaling messages, e.g. inaccordance with ITU standards on SS7. It will be appreciated thatcongestion can arise both from excess voice route utilization which isusing up circuits in those voice routes and from exchange loading whichis a congestion indication which is exchange specific but notnecessarily associated with the voice route which is to go between theAGR network system and the exchange.

Connection control is responsible for ensuring that a connection can bemade between the end points P and Q through the AGR network system.Messages can be passed from call routing (CR) to connection control (CC)which can then eliminate routes with respect to internal congestion, ormay be continually passed from connection control to CR such that callrouting may eliminate candidate routes in priority order. Internalcongestion can be due to having to share the ATM network with othertypes of traffic so that voice traffic demands have to compete withother demands for the same resources, and also because inherently in thedesign of the AGR there is a fragmentation of resources in the physicaldevices. Whilst there may be a certain amount of capacity available on apooled resource, it may in fact be fragmented over a number of physicaldevices and therefore steps should be taken to balance the load betweenthe physical devices. It is possible to arrive at a situation where thecarrying capacity is much less than the design capacity because thiscapacity cannot be accessed fully as a result of fragmentation. In suchcircumstances connection control eliminates the corresponding route andthen passes the message back to call routing for forwarding to adestination AGR.

Thus, the destination AGR choice is based on the highest priorityaccessible, reachable, voice route which immediately identifies the AGRfrom which that voice route emanates so from that point on signalingmessages are forwarded to the outgoing (OG) call routing on the otherside of the network. At that stage the outgoing Call Routing isresponsible for determining whether it can make a particular trunkcircuit selection. A voice route consists of many trunk circuits, socall routing has to deal with the possible race conditions of incomingsignaling on that side which may want to select the same voice circuit,because they are both way circuits, from that same voice route. Theremay be various circuit selection policies. Call routing monitors theprogress of those signals and will then perform the final selection ofthe trunk circuit in that given voice route with respect to whetherthere is a race condition, which is termed glare, and also taking intoaccount whether certain circuits are out of service, not provisioned, orthe type of call, e.g. whether it is a test call. Once the circuit hasbeen selected, call routing passes this information to connectioncontrol and begins the third stage of the process to actually connectthe voice path between the end points P and Q, either in aunidirectional or bidirectional manner in the network system, this beingreferred to as a worm which is a specification of the nodes that thevoice path will pass through. The content of the worm specifies theconnection of the circuit at each stage of the connection in turn fromtrunking AVJ to grooming AVJ and back from grooming AVJ to trunking AVJin the other remote AGR. Essentially the mechanism is driven by the wormbeing passed forward in a chain and then passed back as a form ofacknowledgement. Once it has reached the outgoing stage again then thetrunking AVJ responsible passes it to connection control which passesout a message to Call Routing which can then send out the outgoing IAMsignal. This illustration of the worm is in fact a main path which showsthe whole passage of a call set up. The call handling that isencompassed on the AGR has taken into account external and internalcongestion. When the call routing is a proxy call routing, i.e. the AGRwere just providing a connection entity, it is connection control'sresponsibility to handle internal congestion, and the particular form ofcall routing or special services would take into account their own meansof determining external congestion, for the signaling scheme in use, andthe handshake between Call Routing and connection control and theselection of the outgoing voice route, applies equally to these proxyservers.

FIG. 7 shows how the existing SS7 signaling scheme can be used to signalmessages to and from the AGR network system. In FIG. 7, local exchangeLEA is a local exchange but it is also providing a signaling transferpoint for local exchange LEB, so that local exchange LEB signals itsmessages through local exchange LEA. The AGR network system consists ofAGRs 41 to 44. one of which (41) is the incoming AGR.

Outgoing AGRs are illustrated just to illustrate a means for balancingtraffic load to local exchange LEC which happens to be dual homed ontothese two outgoing AGRs. The local exchange LEA can select a destinationpoint code. The LEA when it has performed digit analysis on the callderives a destination point codes (DPC) and from the destination pointcode derives particular signaling route and then a particular signalinglink set and a particular signaling link to reach the AGR system layerMTP-3 (Message Transfer Part 3) whereby to determine whether themessages are indeed for this AGR, which from the DPC determines whetherthe messages are being forwarded to another exchange, either in the AGRsystem, or external to the AGR system, or whether they have in fact beenrouted incorrectly. From the destination point code and the originatingpoint code, the MPT-3 can tell whether to handle this messageinternally, which is passed to the narrow band ISDN User Part (NIS UP),and determines whether the message came from local exchange LEB or fromlocal exchange LEA, in which case it may be handled accordingly, orwhether the destination point code differs from the AGR in this case,LEC, in which case there is an STP function whereby messages areforwarded to MTP3 in one of the outgoing AGRs using SAAL which is thebroadband signaling ATM adaptation layer. If the destination point codeand originating point code imply a signaling route, the message ispassed to N-ISUP, which from the destination point code and theoriginating point code associates that signaling route with a particularincoming voice route, and the CR entity that takes the signalingmessages and, by digit analysis, determines the possible set ofdestination point codes, i.e. the point code implies a possible set ofoutgoing voice routes that connect between the AGR network system anddestination NB exchanges which therefore implies a possible set ofoutgoing AGRs which head-up those voice routes. Once a choice of voiceroute has been made with due consideration to load balancing, internalcongestion, to determine which is the outgoing AGR, a similar schemefrom Call Routing down through N-ISUP down to MTP3 signals the outgoingexchange, for example the local exchange LEC. Thus, the destinationpoint code of the AGR is now replaced with the destination point code ofexchange LEC, the originating point code is now replaced with that ofthe AGR network system rather than exchange LEA or LEB. The destinationpoint code will imply a particular voice route between that originatingpoint code and the new destination point code, local exchange LEC whichwould imply a particular signaling route and therefore a particularsignaling link set.

Referring now to FIG. 8, this illustrates the way in which call routingpriorities voice routes statically or dynamically according to networkcongestion statistics. Statically means a static prioritization of routeaccording to the three tier network where there is a first priority, asecond priority and so on, or joint first priority, and dynamicallymeans the use of more advanced algorithms such as dynamic call routingalgorithms.

Call routing passes the set of potential destination AGRs to ConnectionControl. Connection Control is aware of the topology of the AGR networksystem, which AGRs are connected together by virtual trunk groups. Theremay be more than one virtual trunk group (VTG) between any given pair ofAGRs. Therefore there is a choice for load balancing between VTGs, butcombined with the candidate set of AGRs there could be more than one VTGto choose from overall. Connection control maps the set of AGRs onto theset of candidate VTGs which are carried in virtual paths in the ATMsense, referred to as virtual trunk routes, and connection control canestablish from the set of VTGs those voice routes which are notreachable due to internal congestion. By knowing the capacity availableon the VTGs, because these are dynamically sized, connection controlknows from congestion criteria whether it can dilate any particular VTGand what the greatest likelihood of success will be for any givenchoice. This provides a facility to block routes which are unreachablein the network at the outset. By rejecting calls with as small amount ofprocessing as possible then, when the system is heavily overloaded thereis reduced processing and signaling generated for calls which can notcurrently be established across the network. By providing thisessentially negative feedback scheme, a call can be rejected at theperiphery of the AGR network before an abortive attempt to route thecall across the network and thus a high degree of carried traffic can bemaintained. Connection control does not necessarily know theprioritization of voice routes, but it will eliminate those voice routeswhich are not reachable without actually changing the priority andtherefore it can achieve a load balancing capability with voice routeswhich are of equal priority or may have changed their dynamic priorityfrom the call routing perspective. Call routing can thus select onegiven route which has a high degree or certainty of success andtherefore need be only indirectly aware of the internal resourceavailability.

A preferred AGR connection protocol stack is illustrated by way ofexample in FIG. 9.

FIG. 10 shows the final stage of the path set up process at which,having selected the outgoing circuit, with knowledge of the incomingcircuit, the voice path is now to be set up therebetween. Because theAGRs have fixed topology, they have an inter-AGR stage which is shown aszone B in this figure. This is the VTG the virtual trunk group andintra-AGR which is the Virtual Channel Junctor VCJ, which is for mostpurposes identical to the VTG and works on substantially identicalprinciples. In this ATM based domain it is necessary to connect from thetrunking AVJs, which interface with trunk circuits directly tointermediate stages which are called grooming AVJs because they groomtraffic from and to the wide area (the inter-AGR traffic) and finally tothe trunking AVJ which is associated with circuit P. The worm thuscontains five identifying features which are illustrated here as being,the outgoing circuit Q, the next stage which identifies a grooming AVJthat will connect us to our chosen VTG, which therefore implies aparticular VCJ, the VTG chosen by Connection Control and the VCJ whichwill take us to the trunking AVJ which interfaces directly with circuitP. By passing this worm forward from controllers built into the AVJ,they can each set up their connection maps to ensure that there is avoice path continuously from circuit P to circuit Q in either aunidirectional or a bidirectional scheme and that could form a forwardor a reverse voice path. Passing the worm to and fro to one end and thenback is a ready form of acknowledgement of the signaling scheme.

FIG. 11 illustrates the functional topology of the system and shows amethod for achieving both load balancing through a number differentroutes between AGRs and also for providing physically diverse routes forresilience purposes. Each AGR consists of trunking AVJs, and groomingAVJs which interface to a wide area network which has an ATM switch.This is a non-blocking ATM switch described earlier. Any local trafficin an AGR would simply be passed between its trunking AVJs. Any widearea network would be passed to a grooming AVJ for grooming on to a VTG.To achieve physically diverse paths one must ensure that those pathspass through a minimum of at least two switches in the ATM network.These switches are the two VC cross-connects shown and since a VTG is avirtual channel connection, it can be cross-connected at the VC leveland by ensuring the ability to connect between any pair of AGRs bypassing through at least two VC crossconnects in the network ensures aphysically diverse path because the crossconnects must be distinct, andtherefore use distinct ports, and therefore there is no single point offailure on those routes. In the path between an AGR and a cross-connectthere is a virtual trunk route (VTR) which in ATM terms is a virtualpath. A VTR can pass through any number of intermediate switches in theATM network functioning as virtual path cross-connects which have notbeen shown for clarity, but logically it is a path from AGR to VCcrossconnect and whatever bandwidth is associated with the virtual trunkroute as it emanates from the AGR is identical to the capacity it hasarrives at the virtual crossconnect. Thus each VTG and its potentiallyphysically diverse pair is contained in at most two VTRs between any twoAGRs. Thus an AGR can determine in its local section of VTR1 to the lefthand side, VTR2 to the right hand side, what capacity has beennegotiated with the ATM network, and what the available bandwidth iscurrently for calls in progress. VTGs are carried within virtual trunkroutes and a virtual trunk route can comprise a number of VTGs, whichare segregated at and multiplexed at the VC cross-connect, hence theutilization of VTR1 is not necessarily the capacity of VTR2, there beingno necessary association. AGR.1 is aware of the capacity and utilizationof VTR1 on the left hand side, and the AGR2 on the right hand side knowsthe capacity and utilization VTR2 then the end to end capacity andutilization is known through the ATM network, whatever the number ofintermediate switching stages, and there is no isolated section whosecapacity and utilization could not be determined in this manner, hencethe AGRNs can determine the congestion level in the network.

Referring now to FIG. 12, this shows a method of determining the sparecapacity on a virtual trunk route. The virtual trunk route statisticsare processed by a VTR capacity monitor which retains the maximum VTRsize which is the bandwidth/capacity negotiated with the ATM network,and the maximum bandwidth that one could use on the route. Some of thisbandwidth may be reserved for maintenance purposes, but the rest isdevoted to VTG bandwidth. The VTR CU also has information on the VTGsthat the VTR comprises and, by subtracting the current absolute sizes ofthe VTGs from the maximum usable bandwidth of the VTR, results in thespare capacity of the VTR, and whether that route is be congested ornot. The VTR CU is ideally located in the Connection Control layer ofthe AGR control system.

A VTR is a shared resource in the Connection Control, and thereforeinterrogation for its spare capacity can be single threaded on a call bycall basis, providing a ready means to share out the spare capacity. TheVTR CU information could be updated every time a call is modified in aVTG; it comprises as a means to reduce signaling, one could have a fixedtime interval, for update, between which the VTR CU grants a credit perinterval to each VTG it comprises based on that VTG's absolute—andchange in utilization in the previous interval, or any other appropriatescheme is applicable, so in this manner the VTR capacity monitor neednot be precise in operation. When the VTR capacity monitor has crossed acertain predefined threshold, and there may be several such thresholds,it flags processed calls a corresponding degree of congestion, andthresholds would be determined according to a desired networkperformance and efficiency. Bandwidth for VTRs may be decided accordingto desired network performance and those thresholds can be used as anindication of internal congestion in place of or in conjunction with theabsolute spare capacity, the indication of having crossed the thresholdmay provide a direct means of comparison and suitability betweencandidate VTRs in routing a call, and as a means for data/signalingcompression.

FIG. 13 illustrates the provision of grooming capacity information inthe AVJ device. Whilst there may be spare capacity on a VTR, the VTGs itcomprises could be controlled by distinct physical devices, i.e.grooming AVJs. Consequently whilst there may be spare capacity in theVTR there may no longer be any capacity on a grooming AVJ that controlsa VTG of interest, i.e. a VTG that is candidate for end-to-end routing.A grooming AVJ can signal call-by-call or, on occasion, the currentsizes of the VTGs that it operates to the respective VTR CUs, such thatthey know the size of VTGs, but a grooming AVJ can also send its sparegrooming capacity resource, and associate that spare resource with therelevant and respective VTGs to which it may apply in unequal measuredue to implementation compromises. Thus, the VTR capacity monitor cancheck the quality of a candidate route to VTG resolution, whether it hascrossed a threshold of congestion or whether any of the VTGs havecrossed a threshold of congestion by lack of capacity on their groomingAVJ. VTGs may include padding to ensure constant end-to-end delay in thenetwork. When some or all the channels in a given VTG are not used, byspecifying some minimum number of channels in a VTG, one ensures maximumend-to-end delay, and if those channels are not used then they arepadded, but because they are padded, adding a channel to the VTG wouldnot in fact increase the VTG size, so that capacity extra to the sparegrooming AVJ capacity. Under those situations the AVJ grooming sparecapacity can be ignored. So now we have a complete picture in the VTR'scapacity monitor of any local congestion.

FIG. 14 illustrates the process of advertising local conditions to thefar end through the VTGs, i.e. the AGRs to which a local AGR isconnected. Advertising local congestion in terms of crossed thresholdsor obsolete utilization can be done on a relatively infrequent basis andit provides the other end with information of the remote VTR sectioncongestion, because being able to dilate and increase a channel in a VTGat the local end is necessary but not a sufficient means to ensure thereis capacity at a remote end. This is a preferred means to ensure to avery high probability that calls will be connected on first attempt bymaking at the outset a proper assessment of whether a particular AGR isreachable by any given path and therefore reject blocked traffic atsource and hence the advantage of no impairment to the amount of carriedtraffic under overload conditions local or remote or universal.

A VTR capacity monitor can advertise its local congestion status interms of thresholds or absolute capacity to all or select remote VTRcapacity monitors to which it is connected, by VTGs through the VTGs itcomprises, by using for example the ATM F5 cell method or any otherequivalent signaling scheme which can be associated with a virtualchannel connection or VTG. A VTRCU can advertise to all those VTRCUs orto a selection, based on any selection criteria, for examplegeographical distance or logical hierarchical placement. The VTGs couldbe given for example a geographic location or, reach or distanceindicator, and thereby the congestion could be sent out as an indicationto only a local area if the distance is below a certain threshold withan associated congestion threshold, thereby only AGRs within a givengeographic locale will get signaled first and as congestion increasesthen it could spread to the wider AGR network system. One can envisageany scheme of selection criteria for any advantageous purpose, which maybe select or universal in application. A VTR capacity monitor obtainsremote VTR congestion indication for any VTG that it comprises and withregard to FIG. 10 a, congestion indications of remote grooming capacityby identical means and criteria from a remote VTR CU or remote groomingAVJ, for any VTG it comprises, and it can supply Connection Control anoverall preference by applying any type of weighted cost function to thelocal and remote congestion indicators and Connection Control can decidethereby the best routing policy with relatively up-to-date localinformation and relatively out-of-date advertised remote informationprior to making any call connection, to select a reachable voice routeand then forwarding the signaling remote AGR where its ConnectionControl may re-examine the best VTG route from the remote perspective,where it will have an up-to-date picture of its own congestion and arelatively up-to-date picture of the emanating AGR at the local end anda more up-to-date picture at the point when the final selection of VTGis made to send the worm back across the network to complete the voicepath connection.

As shown in FIG. 15, the scheme described above with reference to FIG.14 can be extended to the provision of voice band processors in anygiven connection. An example of this would be an echo canceller or halfecho canceller, where the exchange in its call routing from thesignaling will decide, because of the length of the route, or otherrelevant criteria on a need to apply echo cancellation for examplebecause the call has or will cross some certain delay budget. Because agrooming AVJ has a natural loop back capability where it simply isgrooming aggregated local traffic for the wide area network, there istraffic between its ATM domain and its TDM domain and then being mappedback into the ATM domain almost immediately, in performing the groomingfunction. This traffic may be diverted most advantageously through avoice band processor, for example a half echo canceller, connecting tothe grooming AVJs, this being the preferred embodiment although allother placements of echo cancellers or voice bands in the end to endconnection may be applied in the ATM or TDM domain. The capacity of thehalf echo cancellers, because they operate by a paired user port andnetwork port, also represent a resource depleted by utilization. Thegrooming AVJ is aware of and controls the maximum capacity of the halfecho canceller to which it is immediately attached, and duringconfiguration is informed which timeslots the echo canceller isconnected to. Because the grooming AVJ knows the identity of thosetimeslots and whether they have a half echo canceller configured forthem, it can automatically determine what the half echo cancellercapacity is that is connected to it, and it can report this togetherwith its own capacity to the VTR capacity monitor. In the United States,inter-exchange carriers on some of their long-routes always need halfecho cancellers, but they have a statistical distribution of the needfor half echo cancellers. One example would be 30% of all routes havinga need for half echo cancellers. By statistically distributing half echocanceller capability across all the grooming AVJs serving such routes,whatever grooming AVJ is chosen there is a statistical chance of havinghalf echo canceller capability if it is necessary. This has theadvantage over the use of dedicated grooming AVJs for half echocancellers by providing greater utilization efficiency and avoidance ofresonance depletion through fragmentation. Connection control may selecta VTG without congestion and satisfy the criteria of echo cancellerresource if applicable. The half echo cancellers can be readily enabledand disabled by simple loop back control within the grooming AVJ whichis another means of inclusion or exclusion of the half echo canceller inthe path and this is the preferred means of accommodating signalingschemes which introduce echo cancellers at the outset of a call and thenexclude them subsequently through subsequent signaling to ensure optimalplacement closest to called party. The grooming AVJs can optimize thisfunction by finding other timeslot means to loop back which do notutilize the half echo canceller resources. The VTR capacity monitor isthus a single reference for all the resources necessary to connect aparticular voice path through the network system.

Referring now to FIGS. 17 to 20, these show in sequence the means foractually setting up the voice path between trunk circuits P and Q. Thisincorporates a 5 part protocol for signaling over to a cascade of anarbitrary number of multiple stages, Each stage comprises an ingress anda egress process defined with respect to the ATM domain, egress beingtraffic that emanates from the ATM domain into the TDM domain in allcases. The egress process keeps a record of free channel timeslots andtherefore dictates where the offset in any given trunk group, VTG or VCJof a new channel is going to be by determining where a free timeslot isin relation to the other active channels already in the trunk group andtherefore the offset into that group when the new channel is added. Asimilar process applies to the criteria for removal of an existingchannel from the trunk group. Using the offset the egress process cansignal to the ingress process which is responsible for assembly of thattrunk group into ATM cells, and the ingress process is responsible foradding the channel through the dynamic structure data transfer functionof our earlier patent, and the egress process can detect the occurrenceof changes in trunk group size and therefore knows that the channel hasbeen connected and can receive subsequently further changes on thatparticular VTG or VCJ. This operation process, firstly is singlethreaded by VTG or VCJ, and the figure illustrates the two parts of theprocess, firstly signaling an offset from egress to ingress, andsecondly making the change to trunk group size, and forwarding the nextstage's offset as a fully pipelined process in a unidirectional manner,in which, connecting a voice path in the upstream direction from P to Qis achieved by signaling in the reverse direction from Q to P which is apreferred embodiment, because it minimizes the signaling transactionsinvolved, but does not imply the exclusion of signaling in the samedirection as the voice circuit and moreover the unidirectional mechanismmay be used to effect a bidirectional mechanism by signaling first fromQ to P and then in an identical manner for the reverse downstream voicepath signaling from P to Q. Once an egress process has signaled aningress process, the ingress process can transfer signaling to theegress process in the same AVJ for example, the grooming AVJ at stage 2which in turn determines the offset it needs for the VTG between stage 3and 2 and signals to that ingress process in stage 3 and so on throughthe system. The offset may be embodied in a cell referred to as a wormwhich passes to and fro to set up a bidirectional voice path. Provisionis made for AVJ stages 2, 3 and 4 to reject the connection because thecongestion indication given by the VTR capacity monitor could be out ofdate with respect to the true capacity status in any AVJ because ofdelay race for-resource conditions in the AGR network system. Therefore,a backtracking capability is built into the AVJ connection entities andprovision is made for the means to function correctly in the absence ofan advertising mechanism in part or in total. Provision is also made forcorrect operation in the absence of a VTR capacity monitor by abacktrack capability or similar recovery means. The AVJs at stages 1 and2, may optionally have capability to send a digital signature therebyguarding against any false simulation of that identity intentionally orotherwise. This provides a means of authentication such that eachtrunking AVJ at the end of a communication path knows that it hasconnected the voice path through to AGR network correctly providinganalogous integrity checks in existing narrowband exchanges which have,for example, a additional integrity pattern or other signaling schemesto ensure proper cross-connection of narrow band channels through thefabric. We envisage sending a digital signature for unambiguouslyidentifying the identities of the trunking AVJs at each end, and thecircuits P and Q prior to or subsequent to completion of the voice path,in the bandwidth of that voice path on erection, or prior to dissolutionof that voice path in order to provide a means to ensure continualintegrity checking on operations pertaining to network connectivity. Thegrooming AVJs are free to choose any free time slot in their connectionmap. This need not be prescribed by the worm, but the worm can beannotated with that information so that connection control software canreadily use the returned worm acknowledgement to check that the AVJdevices are performing the intended function. This is not a strictrequirement, there still being a means for disconnection of any trunkcircuit Q from any trunk circuit P by a voice path trace operation.

FIG. 18 shows an example of the main contents of the worm. At the top ofthe diagram are illustrated functional stages in which TID meansTrunking Identity, GID means Grooming Identity and the suffix denotesthe local and remote ends. The remote end AGR is the incoming end of thecall, and the local end is the outgoing end of the call since the wormbegins at the outgoing end. The incoming half echo canceller and theoutgoing half echo canceller, are positioned relative to the respectiveends of the call. The incoming side is the left hand side, the remoteside, because call routing forwarded messages to the right hand side andthe preferred embodiment is to send information in the backwardsdirection initially to minimize transactions. The items in heavy typeare those items which are necessary to connect a unique path through thesystem It should be noted here that the remote trunking time slot P andthe remote trunking AVJ identity can be forwarded from the remote end aspart of a call routing signaling message and no other topologyinformation regarding the remote AGR is required. For example, underfault or maintenance a remote AGR could independently change itsconfiguration, and the information forwarded need only be valid on acall-by-call basis. There is a window of opportunity of call-by-callresolution for making configuration changes without affecting any othercalls in progress in the system. The trunking AVJ ID at the local end,the trunking time slot and the grooming identity which heads theselected VTG are all locally stored topological information. Theinclusion or exclusion of echo cancellers is a call-by-call decisionprocess. The central part of FIG. 18 shows that in the AGR networksystem which connects to peripheral exchanges, any signaling forinclusion or exclusion of incoming or outgoing echo cancellers isassociated with the respective side of the call, the exchange on thelocal side only affects incoming echo cancellers. The exchange on theremote side can be arranged to signal only for outgoing half echocancellers and this avoids the need to store any information orconfiguration supervision of echo cancellers in the remote AGR. andlocal information need be retained during the lifetime and dissolutionof a call. The lower portion of FIG. 18 shows three main types of wormoperation namely, the connect, the disconnect and the modify commands.The connect command is for setting up the initial voice path and theitems which are grayed out are those which can be annotated by thegrooming AVJs and stored by connection control if necessary, but whichare not absolutely necessary for the system operation. In total thesecomprise an indication of inclusion or exclusion half echo cancellersand a specification of the exact route that the voice path should take.This facilitates the setting up of point to multipoint connections inthe AGR network system. The disconnect command can optionally includethe local information which is circled in the figure. In apoint-to-multipoint connection, then this information would be needed tospecify unambiguously the grooming AVJ identity and there may be morethan one voice path through that same grooming AVJ, distinguished by thegrooming time slots, which must be passed back from the original connectmessage. In a point-to-point connection using for example SS7 signaling,these circled pieces of information are not necessary because any timeslot P to any other time slot Q has just one unique connection. Adisconnect ion can be facilitated by a bare minimum of information ofjust one end of the connection, for example timeslot Q, and both are notnecessary to be known as described herein. The notepad area is used forannotation as the worm is passed forward, for example to hold the offsetinto the trunk group between each successive AVJ stage. It is distinctfrom the annotated information uniquely identifying the voice path suchas the choice of a grooming time slot. The modify command is associatedwith further subsequent inclusion or exclusion of echo cancellers in thevoice path, which is just one specific example of path modification, alltypes of modification are envisaged, in this particular case it may bedesirable to move a grooming time slot away from a half echo cancellerdevice to a grooming time slot which is a simple loop-back function orvice versa, to free up resources by inverting the original echocanceller indication with an optional facility to specify the groomingtime slot and the worm could be subsequently annotated with the new timeslot, specifying at minimum, the thinking time slot, the grooming AVJidentity, the indication of half echo canceller, and forpoint-to-multipoint connections above the grooming time slots and theVTG.

FIG. 19 shows the way in which a voice path is traced through the AGRnetwork system. The trunking time slot is common to both the ingress andthe egress process for a both-way circuit, and is readily available inthe TDM domain. Hence the trunking time slot for both ingress and egressprocesses with respect to the ATM is known such that, at step 1, thetrunking timeslot location in the egress connection map contains aparticular VCJ identity and the connection entity for that VCJ theoffset of that trunking time slot in the VCJ trunk group, Trunking TimeSlot (T.TS) in the upstream direction. That offset can be passed to thenext AVJ stage, the grooming AVJ. Since VCJs are configured asreciprocating pairs, at step 2, knowing the upstream VCJ also meansknowing the downstream VCJ. The ingress process can then, from the VCJconnection map, determine the offset of the trunking time slot T.T.S(step 3) in that particular VCJ, the offset downstream, offset D. (step4) The offset of the upstream and the downstream direction in thetrunking AVJ have now been determined and can be passed by signaling asdescribed above to the grooming AVJ in the next stage of the connection.At the grooming AVJ, from the offset in the upstream direction in thegiven VCJ, the ingress process at step 2 can determine the groomingtimeslot in the upstream direction (step 3) which is the timeslot usedin the upstream direction on the VCJ side and on the VTG side. From thegrooming timeslot in the upstream direction the egress process candetermine the VTG from the contents of the location in the map and in anidentical manner to the trunking AVJ stage, can determine the offset ofthat grooming timeslot in the VTG trunk group, the offset in theupstream direction (step 3). From the offset in the downstream theegress process can determine the particular grooming timeslot in thedownstream trunk group that occurs at that offset (step 5) andconsequently from complementary pair of the upstream VTG, it determinesthe downstream VTG at step 4. The ingress process can thus determine theoffset in the downstream direction of that grooming timeslot in thatparticular downstream VTG also at step 5, and thus the offset in thedownstream direction and the offset in the upstream direction can besignaled to the next grooming or trunking stage, and the process cancontinue in a cascade in a similar manner so as to determine thecomplete voice path through to the trunk circuit P at the remote end.This provides resilience of the system, for as long as the hardware isreliable. Even where the software could be largely out-of-date, it couldalways recover the hardware state and determine what voice paths areestablished in the network, and resume full operation subsequently.

FIG. 20 shows the overview of the whole wormhole protocol. Step 1 showsthe ITU SS7 message, i.e. the IAM message identifying trunk circuit P.This also contains dialed digits which call routing processes,determines the voice route by consideration for external congestion anda prioritized set of external voice routes (step 2) which it passes toconnection control at step 3. Connection control checks the internalcongestion from the VTR capacity monitors and, if necessary, (step 4),connection control may also initiate any SS7 loop back continuity checkson trunk circuit P or any other type of continuity check appropriate tothe signaling system employed. Connection control eliminates thoseroutes which are unreachable through internal congestion and putsforward its preferred route for load balancing where there is an equalexternal voice route. Call routing determines the final choice of voiceroute and forwards the message. At step 5 the outgoing call routingselects the outgoing trunk circuit and resolves any glare (contentionfor trunk circuits that may be coming in from its own incoming processon this AGR). Once the circuit is selected, connection control checksinternal congestion again through the VTR capacity monitors and makes afinal selection of the VTG that connects the associated pair of AGRs andcreates a worm which comprises sufficient topological information toconnect trunk circuit Q to trunk circuit P via the chosen VTG andforwards this at step 7 to the connection entity inside the trunking AVJhosting Q, which may also initiate a continuity check or any otherscheme provided by the signaling system on trunk circuit Q step 8. Theworm is passed forward through the grooming AVJ stages which mayoptionally include or exclude half echo cancellers in this pathinitially, and they may be excluded at any later stage by a modifiedworm. The grooming AVJ forwards the worm through the VTG to the remoteAGR on the incoming side (step 9) which connects the path through totrunk circuit P and also then passes the worm back in the reversedirection (step 10) to set up in the reverse direction. As a form ofacknowledgement, the annotated information as to what the path was inthe worm information is stored in connection control at both ends sothat the path can be torn down from either end using any of the existingschemes in the signaling system. Once the worm has passed back to theoutgoing side it is passed back to connection control and call routingwhich will then signal an outgoing IAM message (step 11) so as to enablethrough-connections which are shown as switches in FIG. 20. It thusfacilitates the connection of a voice path through this single largedistributed exchange system. Step 12 is an optional stage to check theintegrity of the voice path to enable the through connection and theoutgoing IAM message.

The arrangement and method described provide for the distribution of theexchange in terms of its fabric and its control and its enablingtechnologies with reference to the dynamic structure data transfer.There is uncommitted bandwidth within the fabric and consequentlyrouting decisions can be fully independent of the operation of thisdistributed fabric. The distributed fabric can, because of thisunallocated bandwidth use separate connections to the control layer forestablishing the connections through the fabric and this in no waycompromises external decisions that are made except when an overloadsituation is encountered. An advertising process provides knowledge ofthe distributed fabric. This includes local knowledge about a remotesite such that routing decisions can be made and modified wherenecessary so as to reject traffic at source. The dynamic trunkingenables the separation of call routing and connection control. Thearrangement also provides a means of ensuring stability under overloadsituations and minimizing the cost of handling traffic which would berejected by destinations.

The separation of call routing and connection control together withadvertising the system status ensures a wide range of scaleability sothat the application of dynamic trunking technology providesscaleability in a traffic sense. Further, the separation of call routingand connection control provides a distributed computing environmentwhich is scaleable and managed by this advertisement resource. Becausethe distributed exchange manages its own internal traffic, effectivelyit provides means for balancing that traffic to the fabric and makes itsown internal routing decisions. Once that network grows through itsscaleability, a local exchange can dual home on to it and allow it tomake its own routing decisions. The local exchange does not have to makeany individual routing decisions to a variety of traffic exchanges. Theseparation further provides a facility to support a wide range ofservices using other signaling schemes. The fabric provides a connectionengine that can accommodate a wide variety of signaling protocolsappropriate for the type of service to be provided and that can set upconnections.

The full knowledge of the network connectivity and the release ofresources can be enabled by any node within the network, as it can tracethrough all connections from any starting point. This can be used tosupport failure recovery.

It will be appreciated that although the arrangement and method havebeen described above with particular reference to current standardprotocols, such as SS7 signaling, it is in no way limited to the use ofthese particular protocols.

1. A telecommunications system comprising: a broadband digitalcommunications network; a call routing server configured to determinecall routing information in respect of a call independently of anyconnections being established in the network specifically fortransmitting traffic in respect of the call; and a connection controllerconfigured to establish one or more connections in the network on thebasis of the call routing information, the connections being fortransmitting traffic in respect of the call.
 2. A telecommunicationssystem according to claim 1, wherein the call routing server andconnection controller are configured to communicate with each other inorder to maintain or remove connections in the network in addition toestablishing connections to enable transmission of call traffic oversaid route.
 3. A telecommunications system according to claim 1, whereinthe connection controller is distributed across a plurality of nodes ofthe network.
 4. A telecommunications system according to claim 1,wherein the call routing server is distributed across a plurality ofnodes of the network.
 5. A telecommunications system according to claim1, wherein the call routing server implements narrowband signaling.
 6. Atelecommunications system according to claim 1, wherein the call routingserver is a proxy call routing server.
 7. A telecommunications systemaccording to claim 6, wherein the proxy call routing server uses a proxysignaling protocol to enable said call routing server and connectioncontroller to communicate with each other.
 8. A telecommunicationssystem according to claim 6, wherein the proxy call routing serverimplements a service that goes beyond the routing capabilities ofnarrowband signaling.
 9. A telecommunications system according to claim6, wherein the proxy call routing server is a multimedia server.
 10. Atelecommunications system according to claim 1, wherein the routeidentifies endpoints of the network between which call traffic may betransmitted.
 11. A telecommunications system according to claim 1,wherein the call routing server is arranged to prioritize ones of aplurality of potential call routes according to a static priority.
 12. Atelecommunications system according to claim 1, wherein the call routingserver is arranged to prioritize ones of a plurality of potential callroutes according to a dynamic priority.
 13. A telecommunications systemaccording to claim 1, wherein the call routing server is arranged toselect between a plurality of potential call routes according to anassessment of congestion on the plurality of potential call routes,thereby to balance load across the network.
 14. A telecommunicationssystem according to claim 13, wherein the assessment of congestion ismade using one or more monitors.
 15. A telecommunications systemaccording to claim 14, wherein the one or more monitors are arranged tomonitor the capacity of one or more virtual trunks interconnecting nodesof the network.
 16. A telecommunications system according to claim 14,wherein the one or more monitors are distributed across one or morenodes of the network.
 17. A telecommunications system according to claim14, wherein the one or more monitors are arranged to advertisecongestion status from one or more local to one or more remote nodes ofthe network.
 18. A telecommunications system according to claim 17,wherein the one or more monitors are arranged to advertise congestionstatus to only a selection of nodes of the network.
 19. Atelecommunications system according to claim 18, wherein the selectionof nodes is on the basis of geographical distance from a local node. 20.A telecommunications system according to claim 18, wherein the selectionof nodes is on the basis of logical placement with respect to a localnode.
 21. A telecommunications system according to claim 1, wherein thecall routing server is arranged to determine availability of a calldestination point and to reject a call to that destination point in theevent that the destination point is unavailable.
 22. Atelecommunications system according to claim 21, wherein the destinationpoint is unavailable as a result of a resource on at least part of acall route to the destination being congested.
 23. A telecommunicationssystem according to claim 1, wherein the connection controller isarranged to select one or more of a plurality of potential call routesfor transmitting call traffic on the basis of an assessment ofcongestion in the network, thereby to balance load across the network.24. A telecommunications system according to claim 23, wherein theassessment of congestion is made using one or more monitors.
 25. Atelecommunications system according to claim 24, wherein the one or moremonitors are arranged to monitor the capacity of one or more virtualtrunks interconnecting nodes of the network.
 26. A telecommunicationssystem according to claim 24, wherein the one or more monitors aredistributed across one or more nodes of the network.
 27. Atelecommunications system according to claim 24, wherein the one or moremonitors are arranged to advertise congestion status from one or morelocal to one or more remote nodes of the network.
 28. Atelecommunications system according to claim 27, wherein the one or moremonitors are arranged to advertise congestion status to only a selectionof nodes of the network.
 29. A telecommunications system according toclaim 28, wherein the selection of nodes is on the basis of geographicaldistance from a local node.
 30. A telecommunications system according toclaim 28, wherein the selection of nodes is on the basis of logicalplacement with respect to a local node.
 31. A telecommunications systemaccording to claim 1, wherein the connection controller is arranged toreject a call to be routed through a specified endpoint of the networkif said endpoint is unreachable as a result of an assessment ofcongestion in the network.
 32. A telecommunications system according toclaim 31, wherein the assessment of congestion is made using one or moremonitors.
 33. A telecommunications system according to claim 32, whereinthe one or more monitors are arranged to monitor the capacity of one ormore virtual trunks interconnecting nodes of the network.
 34. Atelecommunications system according to claim 32, wherein the one or moremonitors are distributed across one or more nodes of the network.
 35. Atelecommunications system according to claim 32, wherein the one or moremonitors are arranged to advertise congestion status from one or morelocal to one or more remote nodes of the network.
 36. Atelecommunications system according to claim 35, wherein the one or moremonitors are arranged to advertise congestion status to only a selectionof nodes of the network.
 37. A telecommunications system according toclaim 36, wherein the selection of nodes is on the basis of geographicaldistance from a local node.
 38. A telecommunications system according toclaim 36, wherein the selection of nodes is on the basis of logicalplacement with respect to a local node.
 39. A telecommunications systemaccording to claim 1, comprising a local exchange coupled to thenetwork.
 40. A telecommunications system according to claim 39, whereinthe local exchange is multi homed to the network at a plurality ofnetwork ingress points and wherein one of said ingress points throughwhich to route call traffic from said local exchange into the network isselected in dependence on an assessment of congestion in the networkthereby to balance load across the network.
 41. A telecommunicationssystem according to claim 39, wherein a local exchange is multi homed tothe network at a plurality of network ingress points and wherein one ofsaid ingress points through which to route call traffic from said localexchange into the network is selected in dependence on an assessment ofcongestion in the telecommunications system thereby to balance load onroutes homing to the network.
 42. A telecommunications system accordingto claim 40, wherein the assessment of congestion is made using one ormore monitors.
 43. A telecommunications system according to claim 42,wherein the one or more monitors are arranged to monitor the capacity ofone or more virtual trunks interconnecting nodes of the network.
 44. Atelecommunications system according to claim 42, wherein the one or moremonitors are distributed across one or more nodes of the network.
 45. Atelecommunications system according to claim 42, wherein the one or moremonitors are arranged to advertise congestion status from one or morelocal to one or more remote nodes of the network.
 46. Atelecommunications system according to claim 45, wherein the one or moremonitors are arranged to advertise congestion status to only a selectionof nodes of the network.
 47. A telecommunications system according toclaim 46, wherein the selection of nodes is on the basis of geographicaldistance from a local node.
 48. A telecommunications system according toclaim 46, wherein the selection of nodes is on the basis of logicalplacement with respect to a local node.
 49. A telecommunications systemaccording to claim 1, wherein the network is a packet-switched network.50. A telecommunications system according to claim 1, wherein thenetwork is a connection-oriented network.
 51. A telecommunicationssystem according to claim 50, wherein the network is an asynchronoustransfer mode (ATM) network.
 52. A telecommunications system accordingto claim 1, wherein the network is a wide area network (WAN).
 53. Atelecommunications system according to claim 1, wherein the call routingserver resolves contention for resources.
 54. A telecommunicationssystem according to claim 1, wherein the call routing server responds tochanges in state of a call by communicating with the connectioncontroller, thereby to add, modify or remove connections in the networkfor transmitting call traffic for said call.
 55. A telecommunicationssystem according to claim 1, wherein the call routing server responds tochanges in state of a call by arranging for an echo canceller to be usedin the call.
 56. A telecommunications system according to claim 1,wherein the network carries types of traffic other than said calltraffic.
 57. A telecommunications system according to claim 1, whereinone or more of the connections are unidirectional.
 58. Atelecommunications system according to claim 1, wherein one or more ofthe connections are bidirectional.
 59. A telecommunications systemaccording to claim 1, wherein the connections are each establishedacross one or more virtual trunks interconnecting nodes of the network.60. A telecommunications system according to claim 1, wherein virtualtrunks are arranged to provide physically diverse paths through thenetwork for resiliency.
 61. A node for use in a telecommunicationssystem comprising a broadband digital communications network, the nodecomprising: a call routing server configured to determine call routinginformation in respect of a call independently of any connections beingestablished in the network specifically for transmitting traffic inrespect of the call; and a connection controller configured to establishone or more connections in the network on the basis of the call routinginformation, the connections being for transmitting traffic in respectof the call.
 62. A node according to claim 61, wherein the call routingserver and connection controller are configured to communicate with eachother in order to maintain or remove connections in the network inaddition to establishing connections to enable transmission of calltraffic over said route.
 63. A node according to claim 61, wherein atleast one of the call routing server and the connection controller isdistributed across a plurality of nodes of the network.
 64. A nodeaccording to claim 61,wherein the connection controller is arranged toselect one or more of a plurality of potential call routes fortransmitting call traffic on the basis of an assessment of congestion inthe network, thereby to balance load across the network.
 65. A nodeaccording to claim 61,wherein the connection controller is arranged toreject a call to be routed through a specified endpoint of the networkif said endpoint is unreachable as a result of an assessment ofcongestion in the network.
 66. A node according to claim 61,wherein thecall routing server responds to changes in state of a call bycommunicating with the connection controller, thereby to add, modify orremove connections in the network for transmitting call traffic for saidcall.
 67. A call routing server for use in a telecommunications systemcomprising a broadband digital communications network, the call routingserver comprising: a call routing information determiner configured todetermine call routing information in respect of a call independently ofany connections being established in the network specifically fortransmitting traffic in respect of the call.
 68. A call routing serveraccording to claim 67,wherein the call routing server is configured tocommunicate with a connection controller in the network in order tomaintain or remove connections in the network in addition toestablishing connections to enable transmission of call traffic over aroute determined by the call routing server.
 69. A call routing serveraccording to claim 67, wherein the connection controller is distributedacross a plurality of nodes of the network.
 70. A call routing serveraccording to claim 67, wherein the call routing server implementsnarrowband signaling.
 71. A call routing server according to claim 67,wherein the call routing server is a proxy call routing server.
 72. Acall routing server according to claim 71, wherein the proxy callrouting server uses a proxy signaling protocol to perform saidcommunicating.
 73. A call routing server according to claim 71, whereinthe proxy call routing server implements a service that goes beyond therouting capabilities of narrowband signaling.
 74. A call routing serveraccording to claim 71, wherein the proxy call routing server is amultimedia server.
 75. A call routing server according to claim 67,wherein the route identifies endpoints of the network between which calltraffic may be transmitted.
 76. A call routing server according to claim67, wherein the call routing server is arranged to prioritize ones of aplurality of potential call routes according to a static priority.
 77. Acall routing server according to claim 67, wherein the call routingserver is arranged to prioritize ones of a plurality of potential callroutes according to a dynamic priority.
 78. A call routing serveraccording to claim 67, wherein the call routing server is arranged toselect between a plurality of potential call routes according to anassessment of congestion on the plurality of potential call routes,thereby to balance load across the network.
 79. A call routing serveraccording to claim 78, wherein the assessment of congestion is based onadvertisements of congestion status received from one or more remotenodes of the network.
 80. A call routing server according to claim 67,wherein the call routing server is arranged to determine availability ofa call destination point and to reject a call to that destination pointin the event that the destination point is unavailable.
 81. A callrouting server according to claim 80, wherein the destination point isunavailable as a result of a resource on at least part of a call routeto the destination being congested.
 82. A call routing server accordingto claim 67, wherein the network is a packet-switched network.
 83. Acall routing server according to claim 67, wherein the network is aconnection-oriented network.
 84. A call routing server according toclaim 83, wherein the network is an asynchronous transfer mode (ATM)network.
 85. A call routing server according to claim 67, wherein thenetwork is a wide area network (WAN).
 86. A call routing serveraccording to claim 67, wherein the network carries types of trafficother than said call traffic.
 87. A call routing server according toclaim 67, wherein the call routing server responds to changes in stateof a call by communicating with the connection controller, thereby toadd, modify or remove network for transmitting call traffic for saidcall.
 88. A call routing server according to claim 67, wherein the callrouting server responds to changes in state of a call by arranging foran echo canceller to be used in the call.
 89. A connection controllerfor use in a telecommunications system comprising a broadband digitalcommunications network, the connection controller comprising: aconnection establisher configured to establish one or more connectionsin the network on the basis of predetermined call routing information inrespect of a call, the connections being for transmitting traffic inrespect of the call, the call routing information having been determinedindependently of any connections being established in the networkspecifically for transmitting traffic in respect of the call.
 90. Aconnection controller according to claim 89,wherein the connectioncontroller is configured to communicate with a call routing server inthe network in order to maintain or remove connections in the network inaddition to establishing connections to enable transmission of calltraffic over a route determined by the call routing server.
 91. Aconnection controller according to claim 89, wherein the routeidentifies endpoints of the network between which call traffic may betransmitted.
 92. A connection controller according to claim 89, whereinthe connection controller is arranged to select one of a plurality ofpotential call routes for transmitting call traffic on the basis of anassessment of congestion in the network, thereby to balance load acrossthe network.
 93. A connection controller according to claim 92, whereinthe assessment of congestion is based on advertisements of congestionstatus received from one or more remote nodes of the network.
 94. Aconnection controller according to claim 89, wherein the connectioncontroller is arranged to reject a call to be routed through a specifiedendpoint of the network if said endpoint is unreachable as a result ofan assessment of congestion in the network.
 95. A connection controlleraccording to claim 94, wherein the assessment of congestion is based onadvertisements of congestion status received from one or more remotenodes of the network.
 96. A connection controller according to claim 89,wherein the network is a packet-switched network.
 97. A connectioncontroller according to claim 89, wherein the network is aconnection-oriented network.
 98. A connection controller according toclaim 97, wherein the network is an asynchronous transfer mode (ATM)network.
 99. A connection controller according to claim 89, wherein thenetwork is a wide area network (WAN).
 100. A connection controlleraccording to claim 89, wherein the network carries types of trafficother than said call traffic.
 101. A connection controller according toclaim 89, wherein one or more of the connections are unidirectional.102. A connection controller according to claim 89, wherein one or moreof the connections are bidirectional.
 103. A telecommunications systemcomprising: a broadband digital communications network; means fordetermining call routing information in respect of a call independentlyof any connections being established in the network specifically fortransmitting traffic in respect of the call; and means for establishingone or more connections in the network on the basis of the call routinginformation, the connections being for transmitting traffic in respectof the call.
 104. A node for use in a telecommunications systemcomprising a broadband digital communications network, the nodecomprising: means for determining call routing information in respect ofa call independently of any connections being established in the networkspecifically for transmitting traffic in respect of the call; and meansfor establishing one or more connections in the network on the basis ofthe call routing information, the connections being for transmittingtraffic in respect of the call.
 105. A call routing server for use in atelecommunications system comprising a broadband digital communicationsnetwork, the call routing server comprising: means for determining callrouting information in respect of a call independently of anyconnections being established in the network specifically fortransmitting traffic in respect of the call.
 106. A connectioncontroller for use in a telecommunications system comprising a broadbanddigital communications network, the connection controller comprising:means for establishing one or more connections in the network on thebasis of predetermined call routing information in respect of a call,the connections being for transmitting traffic in respect of the call,the call routing information having been determined independently of anyconnections being established in the network specifically fortransmitting traffic in respect of the call
 107. A method ofestablishing a call in a telecommunications system comprising abroadband digital communications network; the method comprising:determining call routing information in respect of a call independentlyof any connections being established in the network specifically fortransmitting traffic in respect of the call; and establishing one ormore connections in the network on the basis of the call routinginformation, the connections being for transmitting traffic in respectof the call.
 108. A method of determining call routing information foruse in a telecommunications system comprising a broadband digitalcommunications network, the method comprising the steps of: determiningcall routing information in respect of a call independently of anyconnections being established in the network specifically fortransmitting traffic in respect of the call.
 109. A method according toclaim 108, wherein the route identifies endpoints of the network betweenwhich call traffic may be transmitted.
 110. A method according to claim108, wherein the call routing information is determined by a callrouting server and the call routing server prioritizes ones of aplurality of potential call routes according to a static priority. 111.A method according to claim 108, wherein the call routing information isdetermined by a call routing server and the call routing serverprioritizes ones of a plurality of potential call routes according to adynamic priority.
 112. A method according to claim 108, wherein the callrouting information is determined by a call routing server and the callrouting server selects between a plurality of potential call routesaccording to an assessment of congestion on the plurality of potentialcall routes, thereby to balance load across the network.
 113. A methodaccording to claim 112, wherein the assessment of congestion is based onadvertisements of congestion status received by one or more local nodesfrom one or more remote nodes of the network.
 114. A method according toclaim 108, wherein the call routing information is determined by a callrouting server and the call routing server determines availability of acall destination point and rejects a call to that destination point inthe event that the destination point is unavailable.
 115. A methodaccording to claim 114, wherein the destination point is unavailable asa result of a resource on at least part of a call route to thedestination being congested.
 116. A method according to claim 108,wherein a connection controller selects one of a plurality of potentialcall routes for transmitting call traffic on the basis of an assessmentof congestion in the network, thereby to balance load across thenetwork.
 117. A method according to claim 116, wherein the assessment ofcongestion is based on advertisements of congestion status received byone or more local nodes from one or more remote nodes of the network.118. A method according to claim 108, wherein a connection controllerrejects a call to be routed through a specified endpoint of the networkif said endpoint is unreachable as a result of an assessment ofcongestion in the network.
 119. A method according to claim 118, whereinthe assessment of congestion is based on advertisements of congestionstatus received by one or more local nodes from one or more remote nodesof the network.
 120. A method according to claim 108, wherein a localexchange is multi homed to the network at a plurality of network ingresspoints and wherein one of said ingress points through which to routecall traffic from said local exchange into the network is selected independence on an assessment of congestion in the network thereby tobalance load across the network.
 121. A method according to claim 120,wherein the assessment of congestion is based on advertisements ofcongestion status received by one or more local nodes from one or moreremote nodes of the network.
 122. A method according to claim 108,wherein a local exchange is multi homed to the network at a plurality ofnetwork ingress points and wherein one of said ingress points throughwhich to route call traffic from said local exchange into the network isselected in dependence on an assessment of congestion in thetelecommunications system thereby to balance load on routes homing tothe network.
 123. A method according to claim 108, wherein the networkis a packet-switched network.
 124. A method according to claim 108,wherein the network is a connection-oriented network.
 125. A methodaccording to claim 124, wherein the network is an asynchronous transfermode (ATM) network.
 126. A method according to claim 108, wherein thenetwork is a wide area network (WAN).
 127. A method according to claim108, wherein the network carries types of traffic other than said calltraffic.
 128. A method according to claim 108, wherein one or more ofthe connections are unidirectional.
 129. A method according to claim108, wherein one or more of the connections are bidirectional.
 130. Amethod in a telecommunications system comprising a broadband digitalcommunications network, the method comprising: establishing one or moreconnections in the network on the basis of predetermined call routinginformation in respect of a call, the connections being for transmittingtraffic in respect of the call, the call routing information having beendetermined independently of any connections being established in thenetwork specifically for transmitting traffic in respect of the call.131. A telecommunications system comprising: a broadband digitalcommunications network; a call routing server configured to make atleast an initial call routing decision in respect of a callindependently of any connections being established in the networkspecifically for transmitting traffic in respect of the call; and aconnection controller configured to establish one or more connections inthe network for transmitting traffic in respect of the call thereby togive effect to the at least initial call routing decision.
 132. A nodefor use in a telecommunications system comprising a broadband digitalcommunications network, the node comprising: a call routing serverconfigured to make at least an initial call routing decision in respectof a call independently of any connections being established in thenetwork specifically for transmitting traffic in respect of the call;and a connection controller configured to establish one or moreconnections in the network for transmitting traffic in respect of thecall thereby to give effect to the at least initial call routingdecision.
 133. A call routing server for use in a telecommunicationssystem comprising a broadband digital communications network, the callrouting server comprising: an initial call routing decision makerconfigured to make at least an initial call routing decisionindependently of any connections being established in the networkspecifically for transmitting traffic in respect of the call.
 134. Aconnection controller use in a telecommunications system comprising abroadband digital communications network, the connection controllercomprising: a receiver configured to receive an at least initial callrouting decision made independently of any connections being establishedin the network specifically for transmitting traffic in respect of thecall, and a connection establisher configured to establish one or moreconnections in the network for transmitting traffic in respect of thecall thereby to give effect to the at least initial call routingdecision.
 135. A method of routing calls in a telecommunications systemcomprising a broadband digital communications network, the methodcomprising the following steps: making at least initial call routingdecisions in respect of a call independently of any connections beingestablished in the network specifically for transmitting traffic inrespect of the call.
 136. A telecommunications system comprising: abroadband digital communications network, and a connection controllerfor establishing, maintaining, and removing connections in the network,the connections being for transmitting call traffic, wherein theconnection controller is arranged to determine at source a route acrossthe network for a connection and to establish the connection byforwarding a message through the network, the message indicating theroute.
 137. A telecommunications system according to claim 136, whereinthe message indicates the route by indicating one or more virtual trunksinterconnecting nodes of the network.
 138. A telecommunications systemaccording to claim 136, wherein the route is determined in accordancewith an assessment of congestion in the network.
 139. Atelecommunications system according to claim 138, wherein the assessmentof congestion is made using one or more monitors.
 140. Atelecommunications system according to claim 139, wherein the one ormore monitors are arranged to monitor the capacity of one or morevirtual trunks interconnecting nodes of the network.
 141. Atelecommunications system according to claim 139, wherein the one ormore monitors are distributed across one or more nodes of the network.142. A telecommunications system according to claim 139, wherein the oneor more monitors are arranged to advertise congestion status from one ormore local to one or more remote nodes of the network.
 143. Atelecommunications system according to claim 142, wherein the one ormore monitors are arranged to advertise congestion status to only aselection of nodes of the network.
 144. A telecommunications systemaccording to claim 143, wherein the selection of nodes is on the basisof geographical distance from a local node.
 145. A telecommunicationssystem according to claim 138, wherein the selection of nodes is on thebasis of logical placement with respect to a local node.
 146. Atelecommunications system according to claim 136, wherein the network isa packet-switched network.
 147. A telecommunications system according toclaim 136, wherein the network is a connection-oriented network.
 148. Atelecommunications system according to claim 147, wherein the network isan asynchronous transfer mode (ATM) network.
 149. A telecommunicationssystem according to claim 136, wherein the network is a wide areanetwork (WAN).
 150. A telecommunications system according to claim 136,wherein the network carries types of traffic other than said calltraffic.
 151. A telecommunications system according to claim 136,wherein one or more of the connections are unidirectional.
 152. Atelecommunications system according to claim 136, wherein one or more ofthe connections are bidirectional.
 153. A telecommunications systemaccording to claim 136, wherein the connection is established across oneor more virtual trunks interconnecting nodes of the network.
 154. Atelecommunications system according to claim 153, wherein the virtualtrunks are arranged to provide physically diverse paths through thenetwork for resiliency.
 155. A connection controller for use in atelecommunications system comprising a broadband digital communicationsnetwork, the connection controller being for establishing a connectionin the network for transmitting call traffic, the connection controllercomprising: a source route determiner configured to determine at sourcea route across the network for the connection, and a connectionestablisher configured to establish the connection by forwarding amessage through the network, the message indicating the route.
 156. Aconnection controller according to claim 155, wherein the messageindicates the route by indicating one or more virtual trunksinterconnecting nodes of the network.
 157. A connection controlleraccording to claim 155, wherein the route is determined in accordancewith an assessment of congestion in the network.
 158. A connectioncontroller according to claim 155, wherein the network is apacket-switched network.
 159. A connection controller according to claim155, wherein the network is a connection-oriented network.
 160. Aconnection controller according to claim 159, wherein the network is anasynchronous transfer mode (ATM) network.
 161. A connection controlleraccording to claim 155, wherein the network is a wide area network(WAN).
 162. A connection controller according to claim 155, wherein thenetwork carries types of traffic other than said call traffic.
 163. Aconnection controller according to claim 155, wherein one or more of theconnections are unidirectional.
 164. A connection controller accordingto claim 155, wherein one or more of the connections are bidirectional.165. A connection controller according to claim 155, wherein theconnection is established across one or more virtual trunksinterconnecting nodes of the network.
 166. A telecommunications systemcomprising: a broadband digital communications network, and means forestablishing, maintaining, and removing connections in the network, theconnections being for transmitting call traffic, wherein the means forestablishing, maintaining, and removing connections is arranged todetermine at source a route across the network for a connection and toestablish the connection by forwarding a message through the network,the message indicating the route.
 167. A connection controller for usein a telecommunications system comprising a broadband digitalcommunications network, the connection controller being for establishinga connection in the network for transmitting call traffic, theconnection controller comprising: means for determining at source aroute across the network for the connection, and means for establishingthe connection by forwarding a message through the network, the messageindicating the route.
 168. A method of establishing a connection in abroadband digital communications network of a telecommunications system,the connection being for transmitting call traffic, the methodcomprising: determining at source a route across the network for theconnection, and establishing the connection by forwarding a messagethrough the network, the message indicating the route.
 169. A methodaccording to claim 168, wherein the message indicates the route byindicating one or more virtual trunks interconnecting nodes of thenetwork.
 170. A method according to claim 168, wherein the route isdetermined in accordance with an assessment of congestion in thenetwork.
 171. A method according to claim 170, wherein the assessment ofcongestion is made using one or more monitors.
 172. A method accordingto claim 171, wherein the one or more monitors are arranged to monitorthe capacity of one or more virtual trunks interconnecting nodes of thenetwork.
 173. A method according to claim 171, wherein the one or moremonitors are distributed across one or more nodes of the network.
 174. Amethod according to claim 171, wherein the one or more monitors arearranged to advertise congestion status from one or more local to one ormore remote nodes of the network.
 175. A method according to claim 174,wherein the one or more monitors are arranged to advertise congestionstatus to only a selection of nodes of the network.
 176. A methodaccording to claim 175, wherein the selection of nodes is on the basisof geographical distance from a local node.
 177. A method according toclaim 175, wherein the selection of nodes is on the basis of logicalplacement with respect to a local node.
 178. A method according to claim168, wherein the network is a packet-switched network.
 179. A methodaccording to claim 168, wherein the network is a connection-orientednetwork.
 180. A method according to claim 179, wherein the network is anasynchronous transfer mode (ATM) network.
 181. A method according toclaim 168, wherein the network is a wide area network (WAN).
 182. Amethod according to claim 168, wherein the network carries types oftraffic other than said call traffic.
 183. A method according to claim168, wherein one or more of the connections are unidirectional.
 184. Amethod according to claim 168, wherein one or more of the connectionsare bi-directional.
 185. A method according to claim 168, wherein theconnection is established across one or more virtual trunksinterconnecting nodes of the network.
 186. A telecommunications systemcomprising: a broadband digital communications network, and means fordetermining a route through the network for a connection, the connectionbeing for transmitting call traffic; means for establishing a messagespecifying nodes in the network comprising the route of the connectionthrough the network; and means for forwarding said message to each ofthe nodes in the route in a forward direction of said connection andpassing said or another message back through the route by way ofacknowledging establishment of said connection.
 187. A method ofestablishing a connection in a broadband digital communications networkof a telecommunications system, the connection being for transmittingcall traffic, the method comprising: determining a route through thenetwork for the connection; establishing a message specifying nodes inthe network comprising the route of the connection through the network;forwarding said message to each of the nodes in the route in a forwarddirection of said connection and passing said or another message backthrough the route by way of acknowledging establishment of saidconnection.